1. Field of the Invention
The present invention relates generally to ring lasers and more particularly to ring lasers which may be used as gyroscopes to sense rate of rotation. Most particularly, the invention relates to means in the optical cavity of the laser for isolating a pair of plane-polarized, counter-propagating light waves from each other to reduce the coupling between the light waves.
2. Description of the Prior Art
Ring lasers employing electromagnetic waves propagating at optical or near optical frequencies in clockwise and counterclockwise directions about a closed path in a principal plane have been used to sense rates of angular rotation. When this is the case, their function is similar to that of the well-known electromechanical gyro. The rotation of the laser causes a light wave propogating in the direction of rotation to take more time to complete the closed optical path while a light wave propagating in a direction opposite to the direction of rotation takes less times to complete the closed optical path. The increase and decrease in the time required to complete the path corresponds to an increase and decrease, respectively, in the effective length of the closed optical path in the optical cavity for the two light waves.
A basic requirement for sustaining laser oscillation is that the effective closed path length must be an integral number of wavelengths. Consequently, in the presence of rotation, the frequency of each of the counter-propagating light waves must shift to maintain the number of wavelengths in the effective path for each wave an integer. The frequencies of the two waves are then unequal by a difference which is proportional to the rotation rate of the laser.
A problem encountered in the prior art is that the two counter-propagating waves tend to mode lock; that is, for very low differences in the two frequencies, the two waves interfere, or couple, with one another and each assumes a frequency which is identical to the other. This effectively results in a deadband for low angular rates of the gyro. The coupling and consequent mode locking of the two waves is primarily the result of residual backscattering of light from the three or four mirrors which typically define the closed optical path. For example, light from the counterclockwise propagating wave which is backscattered from a corner reflector is ordinarily collinear with the clockwise propagating wave. Similarly, light from the clockwise propagating light wave which is backscattered from a corner reflector, for example, is ordinarily collinear with the counterclockwise propagating wave. Under these conditions, the tendency for mode locking is strongest.
For a description of an improved laser mirror having a reduced tendency to cause backscattering, see U.S. Pat. No. 4,101,707 issued July 18, 1978, to the inventor herein and assigned to Rockwell International Corporation, the assignee herein.
In order to overcome the nonlinearity resulting from mode locking, various means have been employed to bias the laser so as to maintain a frequency difference in excess of that at which mode locking can occur. Laser gyros known to the art commonly employ mechanical rotation or Faraday effect bias to separate the frequencies of counter-propagating laser waves sufficiently to prevent mode locking between the waves, particulary at low rotation rates. Periodic reversal of the bias is employed to minimize sensitivity to bias fluctuations and to provide partial cancellation of mode pulling and backscatter errors. The mechanical bias technique is undesirable from the standpoint of employing moving parts sensitive to the stresses of high acceleration. The Faraday bias method appears more desirable in this respect, but suffers from sensitivity to external magnetic fields and to possible bias reversal asymmetry.